I am thinking of buying a Bushmaster A2 carbine with 16 inch heavy barrel and was wandering what the difference would be in accuracy compared to the 20 inch heavy barrel. I am wanting to be able to shoot no more than 200 yds. As far as group size at this range could I really tell a big difference in accuracy. Are the carbines accurate enough with open sites to varmint hunt at this range?

Barrel length has nothing to do with accuracy. You lose some velocity, but at 200yds., that isnt an issue. Group size has too many variables to think about 4" on the barrel. Group size is determined by, the shooter, the quality of the ammo, the shooter, rifle, and the shooter. If you could see what is done with a service rifle and iron sights at the Nationals you would be amazed. If you are good enough, you can varmint hunt at 200yds with iron sights.

get the 20" barrel....I disagree with there not being much difference in the accuracy between the 16" and the 20" barrels........my son had a 16" and went to a 20" and the difference is just like night and day............

I shot a CMP 100 yard match over the weekend with my 16" Honestly I am not a good enough shot to use this gun to it's full potential.
The 20" does have a longer sight radius, that might make a difference

Like bigbore said.At 200 the 4"will not realy make a difference in accuracy,"i think".BUT at 300 to 500 yea,i belive it will.WHAT ,i want to know is what kind of diff in accuracy at 300 or less is there between a hbar and a standard 1-9 16".Did i just hijack,sorry ,if i did,my bad.Old belfast habit.

At 200 my 14.5 bushy shoots the same as when it was a full size it originaly started out that way 11 years ago and was changed to a more handy model a few months later I had to adjust my front sight a little higher but I think it has more to do with the shorter sight radius other wise I hit the same.

Bigbore is 100% right on despite whatever it is you might hear otherwise. The inherant accuracy potnential of the two are equal. In fact, some argue that the shorter barrels are inherantly more accurate due to less muzzle whip.
The difference in accuracy may be seen in the shorter sight radius on the 16" gun, but that has more to do with the shooters ability to hold the sights straight than the inherant accuracy of the gun.